Catalysed substrate monolith

ABSTRACT

A catalysed substrate monolith  12  for use in treating exhaust gas emitted from a lean-burn internal combustion engine, which catalysed substrate monolith  12  comprising a first washcoat coating  16  and a second washcoat coating  18 , wherein the first washcoat coating comprises a catalyst composition comprising at least one platinum group metal (PGM) and at least one support material for the at least one PGM, wherein at least one PGM in the first washcoat coating is liable to volatilise when the first washcoat coating is exposed to relatively extreme conditions including relatively high temperatures, wherein the second washcoat coating comprises at least one metal oxide for trapping volatilised PGM and wherein the second washcoat coating is oriented to contact exhaust gas that has contacted the first washcoat coating.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority benefit to U.S. Provisional PatentApplication No. 61/569,523 filed on Dec. 12, 2011, and Great BritainPatent Application No. 1200780.3 filed on Jan. 18, 2012, both of whichare incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a catalysed substrate monolith for usein treating exhaust gas emitted from a lean-burn internal combustionengine, particularly vehicular internal combustion engines, whichcatalysed substrate monolith comprising a first washcoat coating and asecond washcoat coating.

BACKGROUND TO THE INVENTION

Generally, there are four classes of pollutant that are legislatedagainst by inter-governmental organisations throughout the world: carbonmonoxide (CO), unburned hydrocarbons (HC), oxides of nitrogen (NO_(x))and particulate matter (PM).

As emissions standards for permissible emission of such pollutants inexhaust gases from vehicular engines become progressively tightened, acombination of engine management and multiple catalyst exhaust gasaftertreatment systems are being proposed and developed to meet theseemission standards. For exhaust systems containing a particulate filter,it is common for engine management to be used periodically (e.g. every500 km) to increase the temperature in the filter in order to combustsubstantially all remaining soot held on the filter thereby to returnthe system to a base-line level. These engine managed soot combustionevents are often called “filter regeneration”. While a primary focus offilter regeneration is to combust soot held on the filter, an unintendedconsequence is that one or more catalyst coatings present in the exhaustsystem, e.g. a filter coating on the filter itself (a so-calledcatalysed soot filter (CSF)) an oxidation catalyst (such as a dieseloxidation catalyst (DOC)) or a NO_(x) adsorber catalyst (NAC) locatedupstream or downstream of the filter (e.g. a first DOC followed by adiesel particulate filter, followed in turn by a second DOC and finallya SCR catalyst) can be regularly exposed to high exhaust gastemperatures, depending on the level of engine management control in thesystem. Such conditions may also be experienced with unintendedoccasional engine upset modes or uncontrolled or poorly controlledregeneration events. However, some diesel engines, particularly heavyduty diesel engines operating at high load, may even expose catalysts tosignificant temperatures, e.g. >600° C. under normal operatingconditions.

As vehicle manufacturers develop their engines and engine managementsystems for meeting the emission standards, the Applicant/Assignee isbeing asked by the vehicle manufacturers to propose catalytic componentsand combinations of catalytic components to assist in the goal ofmeeting the emission standards. Such components include DOCs foroxidising CO, HCs and optionally NO also; CSFs for oxidising CO, HCs,optionally for oxidising NO also, and for trapping particulate matterfor subsequent combustion; NACs for oxidising CO and HC and foroxidising nitrogen monoxide (NO) and absorbing it from a lean exhaustgas and to desorb adsorbed NO_(x) and for reducing it to N₂ in a richexhaust gas (see below); and selective catalytic reduction (SCR)catalysts for reducing NO_(x) to N₂ in the presence of a nitrogenousreductant, such as ammonia (see below).

In practice, catalyst compositions employed in DOCs and CSFs are quitesimilar. Generally, however, a principle difference between the use of aDOC and a CSF is the substrate monolith onto which the catalystcomposition is coated: in the case of a DOC, the substrate monolith istypically a flow-through substrate monolith, comprising a metal orceramic honeycomb monolith having an array of elongate channelsextending therethrough, which channels are open at both ends; a CSFsubstrate monolith is a filtering monolith such as a wall-flow filter,e.g. a ceramic porous filter substrate comprising a plurality of inletchannels arranged in parallel with a plurality of outlet channels,wherein each inlet channel and each outlet channel is defined in part bya ceramic wall of porous structure, wherein each inlet channel isalternately separated from an outlet channel by a ceramic wall of porousstructure and vice versa. In other words, the wall-flow filter is ahoneycomb arrangement defining a plurality of first channels plugged atan upstream end and a plurality of second channels not plugged at theupstream end but plugged at a downstream end. Channels vertically andlaterally adjacent to a first channel are plugged at a downstream end.When viewed from either end, the alternately plugged and open ends ofthe channels take on the appearance of a chessboard.

Quite complicated multiple layered catalyst arrangements such as DOCsand NACs can be coated on a flow-through substrate monolith. Although itis possible to coat a surface of a filter monolith, e.g. an inletchannel surface of a wall-flow filter, with more than one layer ofcatalyst composition, an issue with coating filtering monoliths is toavoid unnecessarily increasing back-pressure, when in use, byoverloading the filter monolith with catalyst washcoat, therebyrestricting the passage of gas therethrough. Hence, although coating asurface of a filter substrate monolith sequentially with one or moredifferent catalyst layers is not impossible, it is more common fordifferent catalyst compositions to be segregated either in zones, e.g.axially segregated front and rear half zones of a filter monolith, orelse by coating an inlet channel of a wall-flow filter substratemonolith with a first catalyst composition and an outlet channel thereofwith a second catalyst composition. However, in particular embodimentsof the present invention, the filter inlet is coated with one or morelayers, which layers may be the same or a different catalystcomposition. It has also been proposed to coat a NAC composition on afiltering substrate monolith (see e.g. EP 0766993).

In exhaust systems comprising multiple catalyst components, eachcomprising a separate substrate monolith, typically, the SCR catalyst islocated downstream of a DOC and/or a CSF and/or a NAC because it isknown that by oxidising some nitrogen oxide (NO) in the exhaust gas tonitrogen dioxide (NO₂) so that there is about a 1:1 ratio of NO:NO₂exiting the DOC and/or the CSF and/or the NAC, the downstream SCRreaction is promoted (see below). It is also well known from EP341832(the so-called Continuously Regenerating Trap or CRT®) that NO₂,generated by oxidising NO in exhaust gas to NO₂, can be used to combustsoot passively on a downstream filter. In exhaust system arrangementswhere the process of EP341832 is important, were the SCR catalyst to belocated upstream of the filter, this would reduce or prevent the processof combusting trapped soot in NO₂, because a majority of the NO_(x) usedfor combusting the soot would likely be removed on the SCR catalyst.

However, a preferred system arrangement for light-duty diesel vehiclesis a diesel oxidation catalyst (DOC) followed by a nitrogenous reductantinjector, then a SCR catalyst and finally a catalysed soot filter (CSF).A short hand for such an arrangement is “DOC/SCR/CSF”. This arrangementis preferred for light-duty diesel vehicles because an importantconsideration is to achieve NO_(x) conversion in an exhaust system asquickly as is possible after a vehicle engine is started to enable (i)precursors of nitrogenous reductants such as ammonia to beinjected/decomposed in order to liberate ammonia for NO_(x) conversion;and (ii) as high NO_(x) conversion as possible. Were a large thermalmass filter to be placed upstream of the SCR catalyst, i.e. between theDOC and the SCR catalyst (“DOC/CSF/SCR”), (i) and (ii) would take farlonger to achieve and NO_(x) conversion as a whole of the emissionstandard drive cycle could be reduced. Particulate removal can be doneusing oxygen and occasional forced regeneration of the filter usingengine management techniques.

It has also been proposed to coat a SCR catalyst washcoat on a filtersubstrate monolith itself (see e.g. WO 2005/016497), in which case anoxidation catalyst may be located upstream of the SCR-coated filtersubstrate (whether the oxidation catalyst is a component of a DOC, a CSFor a NAC) in order to modify the NO/NO₂ ratio for promoting NO_(x)reduction activity on the SCR catalyst. There have also been proposalsto locate a NAC upstream of a SCR catalyst disposed on a flow-throughsubstrate monolith, which NAC can generate NH₃ in situ duringregeneration of the NAC (see below). One such proposal is disclosed inGB 2375059.

NACs are known e.g. from U.S. Pat. No. 5,473,887 and are designed toadsorb NO_(x) from lean exhaust gas (lambda >1) and to desorb the NO_(x)when the oxygen concentration in the exhaust gas is decreased. DesorbedNO_(x) may be reduced to N₂ with a suitable reductant, e.g. engine fuel,promoted by a catalyst component, such as rhodium, of the NAC itself orlocated downstream of the NAC. In practice, control of oxygenconcentration can be adjusted to a desired redox compositionintermittently in response to a calculated remaining NO_(x) adsorptioncapacity of the NAC, e.g. richer than normal engine running operation(but still lean of stoichiometric or lambda=1 composition),stoichiometric or rich of stoichiometric (lambda <1). The oxygenconcentration can be adjusted by a number of means, e.g. throttling,injection of additional hydrocarbon fuel into an engine cylinder such asduring the exhaust stroke or injecting hydrocarbon fuel directly intoexhaust gas downstream of an engine manifold.

A typical NAC formulation includes a catalytic oxidation component, suchas platinum, a significant quantity, (i.e. substantially more than isrequired for use as a promoter such as a promoter in a three-waycatalyst), of a NO_(x)-storage component, such as barium, and areduction catalyst, e.g. rhodium. One mechanism commonly given forNO_(x)-storage from a lean exhaust gas for this formulation is:

NO+1/2O₂→NO₂  (1); and

BaO+2NO₂+1/2O₂→Ba(NO₃)₂  (2),

wherein in reaction (1), the nitric oxide reacts with oxygen on activeoxidation sites on the platinum to form NO₂. Reaction (2) involvesadsorption of the NO₂ by the storage material in the form of aninorganic nitrate.

At lower oxygen concentrations and/or at elevated temperatures, thenitrate species become thermodynamically unstable and decompose,producing NO or NO₂ according to reaction (3) below. In the presence ofa suitable reductant, these nitrogen oxides are subsequently reduced bycarbon monoxide, hydrogen and hydrocarbons to N₂, which can take placeover the reduction catalyst (see reaction (4)).

Ba(NO₃)₂→BaO+2NO+3/2O₂ or Ba(NO₃)₂→BaO+2NO₂+1/2O₂  (3); and

NO+CO→1/2N₂+CO₂  (4);

(Other reactions include Ba(NO₃)₂+8H₂→BaO+2NH₃+5H₂O followed byNH₃+NO→N₂+yH₂O or 2NH₃+2O₂+CO→N₂+3H₂O+CO₂ etc.).

In the reactions of (1)-(4) inclusive herein above, the reactive bariumspecies is given as the oxide. However, it is understood that in thepresence of air most of the barium is in the form of the carbonate orpossibly the hydroxide. The skilled person can adapt the above reactionschemes accordingly for species of barium other than the oxide andsequence of catalytic coatings in the exhaust stream.

Oxidation catalysts promote the oxidation of CO to CO₂ and unburned HCsto CO₂ and H₂O. Typical oxidation catalysts include platinum and/orpalladium on a high surface area support.

The application of SCR technology to treat NO_(x) emissions fromvehicular internal combustion (IC) engines, particularly lean-burn ICengines, is well known. Examples of nitrogenous reductants that may beused in the SCR reaction include compounds such as nitrogen hydrides,e.g. ammonia (NH₃) or hydrazine, or an NH₃ precursor.

NH₃ precursors are one or more compounds from which NH₃ can be derived,e.g. by hydrolysis. Decomposition of the precursor to ammonia and otherby-products can be by hydrothermal or catalytic hydrolysis. NH₃precursors include urea (CO(NH₂)₂) as an aqueous solution or as a solidor ammonium carbamate (NH₂COONH₄). If the urea is used as an aqueoussolution, a eutectic mixture, e.g. a 32.5% NH₃ (aq), is preferred.Additives can be included in the aqueous solutions to reduce thecrystallisation temperature. Presently, urea is the preferred source ofNH₃ for mobile applications because it is less toxic than NH₃, it iseasy to transport and handle, is inexpensive and commonly available.Incomplete hydrolysis of urea can lead to increased PM emissions ontests for meeting the relevant emission test cycle because partiallyhydrolysed urea solids or droplets will be trapped by the filter paperused in the legislative test for PM and counted as PM mass. Furthermore,the release of certain products of incomplete urea hydrolysis, such ascyanuric acid, is environmentally undesirable.

SCR has three main reactions (represented below in reactions (5)-(7)inclusive) which reduce NO_(x) to elemental nitrogen.

4NH₃+4NO+O₂→4N₂+6H₂O (i.e. 1:1 NH₃:NO)  (5)

4NH₃+2NO+2NO₂→4N₂+6H₂O (i.e. 1:1 NH₃:NO_(x))  (6)

8NH₃+6NO₂→7N₂+12H₂O (i.e. 4:3 NH₃:NO_(x))  (7)

A relevant undesirable, non-selective side-reaction is:

2NH₃+2NO₂→N₂O+3H₂O+N₂  (8)

In practice, reaction (7) is relatively slow compared with reaction (5)and reaction (6) is quickest of all. For this reason, when skilledtechnologists design exhaust aftertreatment systems for vehicles, theyoften prefer to dispose an oxidation catalyst element (e.g. a DOC and/ora CSF and/or a NAC) upstream of an SCR catalyst.

When certain DOCs and/or NACs and/or CSFs become exposed to the hightemperatures e.g. encountered during filter regeneration and/or anengine upset event and/or (in certain heavy-duty diesel application)normal high temperature exhaust gas, it is possible given sufficienttime at high temperature for low levels of platinum group metalcomponents, particularly Pt, to volatilise from the DOC and/or the NACand/or the CSF components and subsequently for the platinum group metalto become trapped on a downstream SCR catalyst. This can have a highlydetrimental effect on the performance of the SCR catalyst, since thepresence of Pt leads to a high activity for competing, non-selectiveammonia oxidation such as in reaction (9) (which shows the completeoxidation of NH₃), thereby producing secondary emissions and/orunproductively consuming NH₃.

4NH₃+5O₂→4NO+6H₂O  (9)

One vehicle manufacturer has reported the observation of this phenomenonin SAE paper 2009-01-0627, which is entitled “Impact and Prevention ofUltra-Low Contamination of Platinum Group Metals on SCR catalysts Due toDOC Design” and includes data comparing the NO_(x) conversion activityagainst temperature for a Fe/zeolite SCR catalyst located in seriesbehind four suppliers' platinum group metal (PGM)-containing DOCs thatwere contacted with a flowing model exhaust gas at 850° C. for 16 hours.The results presented show that the NO_(x) conversion activity of aFe/zeolite SCR catalyst disposed behind a 20Pt:Pd DOC at 70 gft⁻³ totalPGM was negatively altered at higher evaluation temperatures as comparedto lower evaluation temperatures as a result of Pt contamination. Two2Pt:Pd DOCs from different suppliers at 105 gft⁻³ total PGM were alsotested. In a first 2Pt:Pd DOC, the SCR catalyst activity was affected toa similar extent as the test on the 20Pt:Pd DOC, whereas for the second2Pt:Pd DOC tested the SCR catalyst activity was contaminated to a lesserextent, although the second 2Pt:Pd DOC still showed reduced NO_(x)conversion activity compared with the blank control (no DOC, just a baresubstrate). The authors concluded that the supplier of the second 2Pt:PdDOC, which showed more moderate NO_(x) conversion degradation, was moresuccessful in stabilising the 70 gft⁻³ Pt present with the 35 gft⁻³ Pd.A Pd-only DOC at 150 gft⁻³ demonstrated no impact on the downstream SCRrelative to the blank control. Earlier work from the authors of SAE2009-01-0627 was published in SAE paper no. 2008-01-2488.

EP 0622107 discloses a catalyst for purifying exhaust gas from dieselengines, wherein platinum catalyst is loaded on the upstream side of anexhaust gas flow, and palladium catalyst is loaded on the lower streamside of the exhaust gas flow. Hydrocarbons (HC) and soluble organicfraction (SOF) in the exhaust gas can be burned and removed by theplatinum catalyst at low temperature. SO₂ is not oxidized at lowtemperature. The exhaust gas is heated to high temperature at theupstream portion. HC and SOF is effectively oxidized and removed bypalladium catalyst at high temperature. SO₂ is not oxidized even athigher temperature. The disclosure claims that in the exhaust gaspurifying catalyst HC and SOF can be removed at low temperature and SO₂is not oxidized.

SUMMARY OF THE INVENTION

Vehicle manufacturers have begun asking the Applicant/Assignee formeasures to solve the problem of volatilisation of relatively low levelsPGMs from components upstream of SCR catalysts. It would be highlydesirable to develop strategies to prevent this PGM movement onto adownstream SCR catalyst at high temperatures. The present inventors havedeveloped a number of strategies for meeting this need.

The inventors have found that volatilisation of platinum from aPGM-containing catalyst comprising both platinum and palladium can occurunder extreme temperature conditions when the weight ratio of Pt:Pd isgreater than about 2:1. It is also believed that where the PGM consistsof platinum, platinum volatilisation may also be observed. The presentinventors have devised a catalysed substrate monolith comprising PGM foruse in combination with a downstream SCR catalyst which avoids orreduces the problem of PGM, particularly Pt, migrating from an upstreamrelatively highly loaded Pt catalyst to a downstream SCR catalyst.

According to a first aspect, the invention provides a catalysedsubstrate monolith for use in treating exhaust gas emitted from alean-burn internal combustion engine, which catalysed substrate monolithcomprising a first washcoat coating and a second washcoat coating,wherein the first washcoat coating comprises a catalyst compositioncomprising at least one platinum group metal (PGM) and at least onesupport material, wherein at least one PGM in the first washcoat coatingis liable to volatilise when the first washcoat is exposed to relativelyextreme conditions including relatively high temperatures, wherein thesecond washcoat comprises at least one metal oxide for trappingvolatilised PGM and wherein the second washcoat is oriented to contactexhaust gas that has contacted the first washcoat coating.

According to a second aspect, the invention provides an exhaust systemfor a lean-burn internal combustion engine, which system comprising afirst catalysed substrate monolith according to the invention.

According to a further aspect, the invention provides a lean-burninternal combustion engine, particularly for a vehicle, comprising anexhaust system according to any preceding claim.

In another aspect the invention provides a vehicle comprising an engineaccording to the invention.

According to another aspect, the invention provides a method of reducingor preventing a selective catalytic reduction (SCR) catalyst in anexhaust system of a lean-burn internal combustion engine from becomingpoisoned with platinum group metal (PGM) which may volatilise from acatalyst composition comprising at least one PGM supported on at leastone support material and disposed on a substrate monolith upstream ofthe SCR catalyst when the catalyst composition comprising PGM is exposedto relatively extreme conditions including relatively high temperatures,which method comprising trapping volatilised PGM in a washcoat coatingcomprising at least one metal oxide, which is disposed on the samesubstrate monolith as the catalyst composition comprising PGM.

A further aspect of the invention relates to the use of a metal oxide(i.e. at least one metal oxide) to reduce or prevent poisoning of aselective catalytic reduction (SCR) catalyst by a platinum group metal(PGM), typically in an exhaust system of a lean-burn internal combustionengine, wherein a second washcoat coating comprises the metal oxide andis oriented to contact exhaust gas that has contacted a first washcoatcoating, and wherein the first washcoat coating comprises a catalystcomposition comprising at least one platinum group metal (PGM) and atleast one support material, and wherein a catalysed substrate monolithcomprises the first washcoat coating and the second washcoat coating. Ingeneral, the metal oxide is for trapping volatilised PGM. Typically, atleast one PGM in the first washcoat coating is liable to volatilise whenthe first washcoat is exposed to relatively extreme conditions includingrelatively high temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully understood, reference ismade to the following Examples by way of illustration only and withreference to the accompanying drawings.

FIG. 1 is a schematic drawing of a laboratory reactor used for testingplatinum contamination on an Fe/Beta zeolite SCR catalyst or a Cu/CHAzeolite SCR catalyst.

FIG. 2 is a graph comparing the NO_(x) conversion activity as a functionof temperature of four aged SCR catalyst cores each of which has beenaged in a laboratory-scale exhaust system configuration containing coresamples of Examples 3, 5 and 6 of the invention or Comparative Example2. The results of the aged SCR activity is plotted against activity of afresh, i.e. un-aged SCR catalyst.

FIG. 3 is a graph comparing the NO_(x) conversion activity as a functionof temperature of a further three aged SCR catalyst cores each of whichhas been aged in a laboratory-scale exhaust system configurationcontaining core samples of Examples 4 and 7 of the invention orComparative Example 2. The results of the aged SCR activity is plottedagainst activity of a fresh, i.e. un-aged SCR catalyst.

FIG. 4 is a graph comparing the NO_(x) conversion activity as a functionof temperature of three aged SCR catalyst cores each of which has beenaged in a laboratory-scale exhaust system configuration containing acatalysed wall-flow filter disposed upstream of the Fe/Beta zeolite SCRcatalyst, one system comprising a filter coated on both inlet and outletchannels with Pt:Pd in a 1:1 weight ratio (Example 7); a second systemcomprising a filter coated on both inlet and outlet channels with aPt:Pd in a 5:1 weight ratio (Example 8); and a third, comparative systemcomprising a filter coated on both inlet and outlet channels with aPt-only catalyst. The results of the aged SCR activity is plottedagainst activity of a fresh, i.e. un-aged SCR catalyst.

FIG. 5 is a bar chart comparing the NO_(x) conversion activity as afunction of temperature of two aged SCR catalyst cores each of which hasbeen aged in the laboratory-scale exhaust system shown in FIG. 1containing core samples of the diesel oxidation catalyst of Example 11heated in a tube furnace at 900° C. for 2 hours in a flowing syntheticexhaust gas with the Cu/CHA zeolite SCR catalyst core held at 300° C.located downstream.

FIGS. 6A and 6B are schematic drawings of exhaust system embodimentsincluding catalysed substrate monoliths according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In general, the at least one PGM in the first washcoat coating comprisesplatinum. When at least one PGM in the first washcoat coating isplatinum, then the platinum is the PGM liable to volatilise when thefirst washcoat coating is exposed to relatively extreme conditionsincluding relatively high temperatures. The relatively extremeconditions including relatively high temperatures are, for example,temperatures of ≧700° C., preferably ≧800° C., or more preferably ≧900°C.

Typically, the PGM in the first washcoat coating comprises both platinumand palladium. The platinum and/or the palladium can be the PGM liableto volatilise when the first washcoat coating is exposed to relativelyextreme conditions including relatively high temperatures. However, whenboth platinum and palladium are present, then normally platinum is morelikely to be the PGM liable to volatilise when the first washcoatcoating is exposed to relatively extreme conditions including relativelyhigh temperatures.

It is possible for higher Pt:Pd weight ratios to be used in the firstwashcoat coating for the purposes of, e.g. generating NO₂ to promotedownstream combustion of filtered particulate matter, because any PGMthat may volatilise from the first washcoat coating in use may betrapped in the second washcoat coating. Typically, the first washcoatcoating comprises a weight ratio of Pt:Pd of ≦10:1, e.g. 8:1, 6:1, 5:1or 4:1.

When the catalysed substrate monolith is disposed immediately upstreamof a SCR catalyst (i.e. without any intervening substrate monolithbetween the catalysed substrate monolith of the present invention andthe SCR catalyst), it is preferred that the weight ratio of Pt:Pd is ≦2,preferably in the first washcoat coating or in the catalysed substratemonolith as whole (i.e. overall). Where the at least one PGM in thefirst washcoat coating comprises both platinum and palladium, preferablythe weight ratio of Pt:Pd is ≦2, such as ≦1.5:1, e.g. about 1:1. Thesignificance of this feature is shown in the Examples: the inventorshave found that the preferred Pt:Pd weight ratios volatilise less, byempiric testing, than a similar catalyst having a Pt:Pd weight ratio of4:1. In layered catalyst arrangements, it is preferred that an outerlayer has a Pt:Pd weight ratio of ≦2, or optionally that the overallPt:Pd weight ratio of all layers combined is ≦2.

Typically, the weight ratio of Pt:Pd in the first washcoat coating oroverall is ≧35:65 (e.g. ≧7:13). It is preferred that the weight ratioPt:Pd is ≧40:60 (e.g. ≧2:3), more preferably ≧45:55 (e.g. ≧9:11),particularly ≧50:50 (e.g. ≧1:1), such as ≧1.25:1, and still morepreferably ≧1.5:1 (e.g. ≧1.6:1). The weight ratio of Pt:Pd, either inthe first washcoat coating or overall, is typically 10:1 to 7:13. It ispreferred that the weight ratio of Pt:Pd is 8:1 to 2:3, more preferably6:1 to 9:11, even more preferably 5:1 to 1:1, such as 4:1 to 1.25:1, andstill more preferably 2:1 to 1.25:1 (e.g. 2:1 to 1.6:1).

Generally, the total amount of the platinum group metal (PGM) (e.g. thetotal amount of Pt and/or Pd) is 1 to 500 g ft⁻³. Preferably, the totalamount of the PGM is 5 to 400 g ft⁻³, more preferably 10 to 300 g ft⁻³,still more preferably, 25 to 250 g ft⁻³, and even more preferably 35 to200 g ft⁻³.

In general, when the catalysed substrate monolith of the presentinvention comprises platinum, then the platinum is not doped withbismuth and/or manganese. More preferably, the catalyst substratemonolith does not comprise bismuth and/or manganese.

In general, the metal oxide (i.e. the at least one metal oxide supportof the second washcoat coating) comprises a metal oxide selected fromthe group consisting of optionally stabilised alumina, amorphoussilica-alumina, optionally stabilised zirconia, ceria, titania, anoptionally stabilised ceria-zirconia mixed oxide and mixtures of any twoor more thereof. Suitable stabilisers include one or more of silica andrare earth metals.

The metal oxide of the second washcoat coating and the at least onesupport material of the first washcoat coating may be the same ordifferent. It is preferred that the metal oxide of the second washcoatcoating and the at least one support material of the first washcoatcoating are different.

The second washcoat coating may, typically, comprise the metal oxide ina total amount of 0.1 to 5 g in⁻³, preferably 0.2 to 4 g in⁻³ (e.g. 0.5to 3.5 g in⁻³), more preferably 1 to 2.5 g in⁻³.

The inventors have found that particularly alumina and ceria-containingmetal oxides per se are capable of trapping volatilised PGMs,particularly ceria, which has a particular affinity for Pt. It ispreferred that the metal oxide of the second washcoat coating isselected from the group consisting of optionally stabilised alumina,ceria and an optionally stabilised ceria-zirconia mixed oxide. Morepreferably, the metal oxide is selected from the group consisting ofoptionally stabilised alumina and an optionally stabilisedceria-zirconia mixed oxide.

In one embodiment, the second washcoat coating does not comprisepalladium and platinum. More preferably, the second washcoat coatingdoes not comprise a platinum group metal (PGM).

In other embodiments, the second washcoat coating may further comprise acatalyst composition comprising at least one metal selected from thegroup consisting of palladium, silver, gold and combinations of any twoor more thereof, wherein the at least one metal oxide supports the atleast one metal. It is preferred that the second washcoat comprises asupported combination of palladium and gold e.g. as an alloy, asdescribed in Applicant/Assignee's WO 2009/136206.

When the second washcoat coating comprises a catalyst compositioncomprising palladium and gold (e.g. as an alloy), then typically thepalladium and gold are not doped with bismuth and/or manganese. Morepreferably, the second washcoat coating does not comprise bismuth and/ormanganese.

Typically, the total amount of the at least one metal in the secondwashcoat coating is from 10 to 350 g ft⁻³. It is preferred that thetotal amount is 20 to 300 g ft⁻³, more preferably 30 to 250 g ft⁻³,still more preferably, 45 to 200 g ft³, and even more preferably 50 to175 g ft⁻³.

When the second washcoat coating comprises a catalyst compositioncomprising palladium, then preferably the second washcoat coating doesnot comprise platinum.

In general, the second washcoat coating is substantially devoid of (i.e.does not comprise) copper and/or rhodium.

The only PGM present in the second washcoat coating is generallypalladium. However, in a particular embodiment, the second washcoatcoating comprises platinum and palladium.

Typically, the weight ratio of Pt:Pd in the second washcoat coating islower than the weight ratio of Pt:Pd in the first washcoat coating (i.e.the relative amount of Pt to Pd in the second washcoat coating is lowerthan the relative amount of Pt to Pd in the first washcoat coating). Thepresent inventors have found that palladium, or a Pt/Pd catalyst havinga relatively high Pd content can act to trap volatilised Pt.

The first washcoat coating comprises a catalyst composition comprisingat least one platinum group metal (PGM) and at least one supportmaterial for the at least one PGM. The catalyst is typically applied tothe substrate monolith as a washcoat slurry comprising at least one PGMsalt and one or more support materials in the finished catalyst coating,before the coated filter is dried and then calcined. The one or moresupport materials may be referred to as a “washcoat component”. It isalso possible for at least one PGM to be pre-fixed to one or moresupport materials prior to it being slurried, or for a combination ofsupport material particles to which PGM is pre-fixed to be slurried in asolution of PGM salt.

By at least one “support material” herein, we mean a metal oxideselected from the group consisting of optionally stabilised alumina,amorphous silica-alumina, optionally stabilised zirconia, ceria,titania, an optionally stabilised ceria-zirconia mixed oxide, amolecular sieve and mixtures or combinations of any two or more thereof.

Typically, the at least one support material of the first washcoatcoating is selected from the group consisting of optionally stabilisedalumina, amorphous silica-alumina, optionally stabilised zirconia,ceria, titania, an optionally stabilised ceria-zirconia mixed oxide, amolecular sieve and mixtures or combinations of any two or more thereof.It is preferred that the first washcoat coating comprises at least onesupport material selected from the group consisting of optionallystabilised alumina, amorphous silica-alumina, ceria and mixtures orcombinations of any two or more thereof.

The at least one support material may include one or more molecularsieve, e.g. an aluminosilicate zeolite. The primary duty of themolecular sieve in the PGM catalyst for use in the present invention isfor improving hydrocarbon conversion over a duty cycle by storinghydrocarbon following cold start or during cold phases of a duty cycleand releasing stored hydrocarbon at higher temperatures when associatedplatinum group metal catalyst components are more active for HCconversion. See for example Applicant/Assignee's EP 0830201. Molecularsieves are typically used in catalyst compositions according to theinvention for light-duty diesel vehicles, whereas they are rarely usedin catalyst compositions for heavy duty diesel applications because theexhaust gas temperatures in heavy duty diesel engines mean thathydrocarbon trapping functionality is generally not required.

However, molecular sieves such as aluminosilicate zeolites are notparticularly good supports for platinum group metals because they aremainly silica, particularly relatively higher silica-to-aluminamolecular sieves, which are favoured for their increased thermaldurability: they may thermally degrade during ageing so that a structureof the molecular sieve may collapse and/or the PGM may sinter, givinglower dispersion and consequently lower HC and/or CO conversionactivity. Accordingly, it is preferred that the first washcoat coatingand/or the second washcoat coating comprise a molecular sieve at ≦30% byweight (such as ≦25% by weight, ≦20% by weight e.g. ≦15% by weight) ofthe individual washcoat coating layer. The remaining at least onesupport material of the first washcoat coating and/or the secondwashcoat coating may comprise a metal oxide selected from the groupconsisting of optionally stabilised alumina, amorphous silica-alumina,optionally stabilised zirconia, ceria, titania, an optionally stabilisedceria-zirconia mixed oxide and mixtures of any two or more thereof.

Preferred molecular sieves for use as support materials/hydrocarbonadsorbers are medium pore zeolites, preferably aluminosilicate zeolites,i.e. those having a maximum ring size of eight tetrahedral atoms, andlarge pore zeolites (maximum of ten tetrahedral atoms) preferablyaluminosilicate zeolites, include natural or synthetic zeolites such asfaujasite, clinoptilolite, mordenite, silicalite, ferrierite, zeolite X,zeolite Y, ultrastable zeolite Y, ZSM-5 zeolite, ZSM-12 zeolite, SSZ-3zeolite, SAPO-5 zeolite, offretite or a beta zeolite, preferably ZSM-5,beta and Y zeolites. Preferred zeolite adsorbent materials have a highsilica to alumina ratio, for improved hydrothermal stability. Thezeolite may have a silica/alumina molar ratio of from at least about25/1, preferably at least about 50/1, with useful ranges of from about25/1 to 1000/1, 50/1 to 500/1 as well as about 25/1 to 100/1, 25/1 to300/1, from about 100/1 to 250/1.

The first washcoat coating can be disposed in a range of configurationsrelative to the second washcoat coating. The first washcoat coating maybe disposed in a first washcoat zone of the substrate monolith and thesecond washcoat may be disposed in a second washcoat zone of thesubstrate monolith, wherein there is substantially no overlap betweenthe first washcoat zone and the second washcoat zone (e.g. there is nooverlap between the first washcoat coating and the second washcoatcoating). Generally, the first washcoat zone is disposed at an inlet endof the catalysed substrate monolith and the second washcoat zone isdisposed at an outlet end of the catalysed substrate monolith.

Alternatively, or in addition, the second washcoat coating may bedisposed in a layer above the first washcoat coating. Of course, whenthe first washcoat coating and the second washcoat coating are disposedon a filter, care has to be taken with any layering arrangement so thatthe “second washcoat coating is oriented to contact exhaust gas that hascontacted the first washcoat coating” feature of the invention is met,e.g. it may be necessary to reverse the orientation of the first andsecond washcoat coating layers applied to outlet channels of a wall-flowfilter.

The substrate monolith for use in the invention can be a flow-throughsubstrate monolith or a filtering substrate monolith. The secondwashcoat coating is generally oriented to contact exhaust gas that hascontacted the first washcoat. This is to allow the first washcoatcoating to come into contact with the exhaust gas first. The exhaust gasand any volatilised PGM from the first washcoat coating is thencontacted with the second washcoat coating that includes a metal oxidefor trapping the volatilised PGM.

A filtering substrate monolith typically has inlet surfaces and outletsurfaces, wherein the inlet surfaces are separated from the outletsurfaces by a porous structure. It is preferred that the filteringsubstrate monolith is a wall-flow filter, i.e. a ceramic porous filtersubstrate comprising a plurality of inlet channels arranged in parallelwith a plurality of outlet channels, wherein each inlet channel and eachoutlet channel is defined in part by a ceramic wall of porous structure,wherein each inlet channel is alternatingly separated from an outletchannel by a ceramic wall of porous structure and vice versa. In otherwords, the wall-flow filter is a honeycomb arrangement defining aplurality of first channels plugged at an upstream end and a pluralityof second channels not plugged at the upstream end but plugged at adownstream end. Channels vertically and laterally adjacent to a firstchannel are plugged at a downstream end. When viewed from either end,the alternately plugged and open ends of the channels take on theappearance of a chessboard.

Catalysed filters, preferably wall-flow filters, can be coated using themethod disclosed in Applicant/Assignee's WO 2011/080525. That is, amethod of coating a honeycomb monolith substrate comprising a pluralityof channels with a liquid comprising a catalyst component, which methodcomprising the steps of: (i) holding a honeycomb monolith substratesubstantially vertically; (ii) introducing a pre-determined volume ofthe liquid into the substrate via open ends of the channels at a lowerend of the substrate; (iii) sealingly retaining the introduced liquidwithin the substrate; (iv) inverting the substrate containing theretained liquid; and (v) applying a vacuum to open ends of the channelsof the substrate at the inverted, lower end of the substrate to draw theliquid along the channels of the substrate. The catalyst composition maybe coated on filter channels from a first end, following which thecoated filter can be dried.

Methods of making catalysed substrate monoliths, including single layerwashcoat coatings and dual layered arrangements (one washcoat coatinglayer above another washcoat coating layer) are known in the art andinclude Applicant/Assignee's WO 99/47260, i.e. comprising the steps of(a) locating a containment means on top, first end of a substratemonolith, (b) dosing a pre-determined quantity of a first washcoatcoating component into said containment means, either in the order (a)then (b) or (b) then (a), and (c) by applying pressure or vacuum,drawing said first washcoat coating component into at least a portion ofthe substrate monolith, and retaining substantially all of said quantitywithin the substrate monolith. In a first step a coating from a firstend of application can be dried and the dried substrate monolith can beflipped through 180 degrees and the same procedure can be done to a top,second end of the substrate monolith, with substantially no overlap inlayers between applications from the first and second ends of thesubstrate monolith. The resulting coating product is then dried, andthen calcined. The process is repeated with a second washcoat coatingcomponent, to provide a catalysed (bi-layered) substrate monolithaccording to the invention.

Use of such a method can be controlled using, e.g. vacuum strength,vacuum duration, washcoat viscosity, washcoat solids, coating particleor agglomerate size and surface tension so that catalyst is coatedpredominantly on the inlet surfaces but also optionally within theporous structure but near to the inlet surfaces. Alternatively, thewashcoat components may be milled to a size, e.g. D90 <5 μm, so thatthey “permeate” the porous structure of the filter (see WO 2005/016497).

It is preferred that the catalysed substrate monolith comprises afiltering substrate monolith (e.g. the catalysed substrate monolith is acatalysed filtering substrate monolith) and a zoned arrangement of thefirst washcoat coating and the second washcoat coating. More preferably,a first washcoat zone comprises inlet surfaces of the filteringsubstrate monolith and the second washcoat zone comprises outletsurfaces of the filtering substrate monolith. In this context, the inletsurfaces generally refer to the walls of the channels of the filteringsubstrate monolith into which exhaust gas enters, and the outletsurfaces generally refer to the walls of the channels of the filteringsubstrate monolith through which the exhaust gas leaves. Thus, forexample, the porous structure or walls separating the inlet and outletsurfaces defines a transition between the first washcoat zone and thesecond washcoat zone.

The first washcoat coating can comprise an oxidation catalyst or aNO_(x) adsorber catalyst (NAC), as described in the background of theinvention hereinabove. A NAC contains significant quantities of alkalineearth metals and/or alkali metals relative to an oxidation catalyst. TheNAC typically also includes ceria or a ceria-containing mixed oxide,e.g. a mixed oxide of cerium and zirconium, which mixed oxide optionallyfurther including one or more additional lanthanide or rare earthelements.

In addition to the first washcoat coating and the second washcoatcoating, the catalysed substrate monolith of the invention may furthercomprise additional washcoat coatings. However, it is preferred that thecatalysed substrate monolith of the invention has only two washcoatcoatings, the first washcoat coating and the second washcoat coating.Thus, the catalysed substrate monolith consists of a first washcoatcoating and a second washcoat coating.

The invention also relates to an exhaust system. The exhaust systempreferably further comprises a second catalysed substrate monolithcomprising a selective catalytic reduction (SCR) catalyst, which secondcatalysed substrate monolith is disposed downstream from the firstcatalysed substrate monolith. An optionally catalysed filteringsubstrate monolith (i.e. a third, optionally catalysed, substratemonolith) can be disposed downstream from the second catalysed substratemonolith (e.g. an exhaust system in a DOC/SCR/CSF arrangement discussedin connection with the background to the invention hereinabove). Thefiltering substrate monolith (i.e. the third, optionally catalysed,substrate monolith) is preferably a wall-flow filter. Where catalysed,the catalyst for use in connection with the filtering substrate monolithis an oxidation catalyst, but in alternative embodiments it can be a NACcomposition. Alternatively, the filtering substrate monolith can beuncatalysed.

Typically, the exhaust system of the invention comprises an injector forinjecting a nitrogenous reductant into exhaust gas between the firstcatalysed substrate monolith and the second catalysed substratemonolith. Alternatively, (i.e. without means for injecting a nitrogenousreductant, such as ammonia or a precursor thereof, such as urea, isdisposed between the first catalysed substrate monolith and the secondcatalysed substrate monolith), or in addition to the means for injectinga nitrogenous reductant (e.g. ammonia or a precursor thereof, such asurea), engine management means may be provided for enriching exhaust gassuch that ammonia gas is generated in situ by reduction of NO_(x) on thecatalyst composition of the first washcoat coating and/or a substratemonolith comprising a DOC or NAC disposed upstream of the firstsubstrate monolith or downstream of the first substrate monolith. Wherethe substrate monolith comprising the DOC or the NAC is disposeddownstream of the filter, preferably it is disposed upstream of themeans for injecting ammonia or a precursor thereof.

Nitrogenous reductants and precursors thereof for use in the presentinvention include any of those mentioned hereinabove in connection withthe background section. Thus, for example, the nitrogenous reductant ispreferably ammonia or urea.

In combination with an appropriately designed and managed dieselcompression ignition engine, enriched exhaust gas, i.e. exhaust gascontaining increased quantities of carbon monoxide and hydrocarbonrelative to normal lean running mode, contacts the NAC. Componentswithin a NAC such as PGM-promoted ceria or ceria-zirconia can promotethe water-gas shift reaction, i.e. CO_((g))+H₂O_((v))→CO_(2(g))+H_(2(g))evolving H₂. From the side reaction footnote to reactions (3) and (4)set out hereinabove, e.g. Ba(NO₃)₂+8H₂→BaO+2NH₃+5H₂O, NH₃ can begenerated in situ and stored for NO_(x) reduction on the downstream SCRcatalyst.

When the first catalysed substrate monolith is a filtering substratemonolith (e.g. a catalysed wall-flow filter), the exhaust systempreferably further comprises a third catalysed substrate monolith,wherein the third catalysed substrate monolith is a flow-throughsubstrate monolith comprising an oxidation catalyst, e.g. a DOC or aNAC, which third catalysed substrate monolith is disposed upstream ofthe first catalysed substrate monolith.

The second catalysed substrate monolith typically comprises a catalystfor selectively catalysing the reduction of oxides of nitrogen todinitrogen with a nitrogenous reductant, also known as a selectivecatalytic reduction (SCR) catalyst. The SCR catalyst may be coated as acoating onto a substrate monolith, such as described hereinabove.Alternatively, the SCR catalyst may be provided as an extrudate (alsoknown as a “catalyst body”), i.e. the catalyst is mixed with componentsof the substrate monolith structure, which are both extruded, so thecatalyst is part of the walls of the substrate monolith.

The SCR catalyst of the second substrate monolith typically comprises afiltering substrate monolith or a flow-through substrate monolith. It isalso possible to make a wall-flow filter from an extruded SCR catalyst(see Applicant/Assignee's WO 2009/093071 and WO 2011/092521). SCRcatalysts can be selected from the group consisting of at least one ofCu, Hf, La, Au, In, V, lanthanides and Group VIII transition metals,such as Fe, supported on a refractory oxide or molecular sieve. Suitablerefractory oxides include Al₂O₃, TiO₂, CeO₂, SiO₂, ZrO₂ and mixed oxidescontaining two or more thereof. Non-zeolite catalyst can also includetungsten oxide, e.g. V₂O₅/WO₃/TiO₂. Preferred metals of particularinterest are selected from the group consisting of Ce, Fe and Cu.Molecular sieves can be ion-exchanged with the above metals.

It is preferred that the at least one molecular sieve, is analuminosilicate zeolite or a SAPO. The at least one molecular sieve canbe a small, a medium or a large pore molecular sieve, for example. By“small pore molecular sieve” herein we mean a molecular sievescontaining a maximum ring size of 8 tetrahedral atoms, such as CHA; by“medium pore molecular sieve” herein we mean a molecular sievecontaining a maximum ring size of 10 tetrahedral atoms, such as ZSM-5;and by “large pore molecular sieve” herein we mean a molecular sievehaving a maximum ring size of 12 tetrahedral atoms, such as beta. Smallpore molecular sieves are potentially advantageous for use in SCRcatalysts—see for example Applicant/Assignee's WO 2008/132452. Molecularsieves for use in SCR catalysts according to the invention include oneor more metals incorporated into a framework of the molecular sieve e.g.Fe “in-framework” Beta and Cu “in-framework” CHA.

Particular molecular sieves with application in the present inventionare selected from the group consisting of AEI, ZSM-5, ZSM-20, ER1including ZSM-34, mordenite, ferrierite, BEA including Beta, Y, CHA, LEVincluding Nu-3, MCM-22 and EU-1, with CHA molecular sieves, e.g.aluminosilicate CHA, currently preferred, particularly in combinationwith Cu as promoter, e.g. ion-exchanged.

The present invention also relates to a lean-burn internal combustionengine. The lean-burn internal combustion engine can be a positiveignition, e.g. a spark ignition, engine that typically run on gasolinefuel or blends of gasoline fuel and other components such as ethanol,but is preferably a compression ignition, e.g. a diesel-type engine.Lean-burn internal combustion engines include homogenous chargecompression ignition (HCCI) engines, powered either by gasoline etc.fuel or diesel fuel.

An exhaust system of the present invention is shown in FIG. 6A. Exhaustsystem 10 comprises, in serial arrangement from upstream to downstream,a catalysed wall-flow filter 2; and a wall-flow filter substratemonolith 4 coated with a Cu/CHA SCR catalyst. Each substrate monolith 2,4 is disposed in a metal container or “can” including coned diffusersand they are linked by a series of conduits 3 of smaller cross sectionalarea than a cross sectional area of any of substrate monoliths 2, 4. Theconed diffusers act to spread the flow of exhaust gas entering a housingof a “canned” substrate monolith so that the exhaust gas as a whole isdirected across substantially the whole front “face” of each substratemonolith. Exhaust gas exiting substrate monolith 4 is emitted toatmosphere at “tail pipe” 5.

Catalysed wall-flow filter 2 is coated with a NO_(x) absorber catalyst(NAC) composition in a zone 6 on inlet channels thereof and palladiumsupported on particulate alumina in a zone 8 on outlet channels thereof.In combination with an appropriately designed and managed dieselcompression ignition engine (upstream of substrate monolith 2, notshown), enriched exhaust gas, i.e. exhaust gas containing increasedquantities of carbon monoxide and hydrocarbon relative to normal leanrunning mode, contacts the NAC. Components within a NAC such asPGM-promoted ceria or ceria-zirconia can promote the water-gas shiftreaction, i.e. CO_((g))+H₂O_((v))→CO_(2(g))+H_(2(g)) evolving H₂. Fromthe side reaction footnote to reactions (3) and (4) set out hereinabove,e.g. Ba(NO₃)₂+8H₂→BaO+2NH₃+5H₂O, NH₃ can be generated in situ and storedfor NO_(x) reduction on the downstream SCR catalyst.

FIG. 6B shows an alternative embodiment of an exhaust system 20according to the present invention comprising, in serial arrangementfrom upstream to downstream, a catalysed flow-through substrate monolith12; a source of ammonia 13 comprising an injector for an ammoniaprecursor, urea; and a flow-through substrate monolith 14 coated with aFe/Beta SCR catalyst. Each substrate monolith 12, 14 is disposed in ametal container or “can” including coned diffusers and they are linkedby a series of conduits 3 of smaller cross sectional area than a crosssectional area of any of substrate monoliths 12, 14. Exhaust gas exitingsubstrate monolith 14 is emitted to atmosphere at “tail pipe” 5.

Catalysed flow-through substrate monolith 12 comprises a first zone 16defined in part by an upstream end thereof coated with a 4:1 Pt:Pdweight ratio catalyst wherein the Pt and Pd are supported on a supportmaterial; and a second zone 18 of about 50% of a total length of theflow-through substrate monolith with substantially no overlap with firstzone 16, which second zone 18 comprising a two-layered arrangementwherein a first (or bottom) layer comprises platinum supported onalumina and a second (or top) layer comprising palladium supported onalumina. The catalysed flow-through substrate monolith is designed forthe purpose of promoting reaction (1) and thereby reaction (6) on thedownstream SCR catalyst.

EXAMPLES Example 1 Preparation of Substrate Monolith Coated with 5 wt %Fe/Beta Zeolite

Commercially available Beta zeolite was added to an aqueous solution ofFe(NO₃)₃ with stirring. After mixing, binders and rheology modifierswere added to form a washcoat composition.

A 400 cells per square inch (cpsi) cordierite flow-through substratemonolith was coated with an aqueous slurry of the 5 wt % Fe/Beta zeolitesample using the method disclosed in Applicant/Assignee's WO 99/47260,i.e. comprising the steps of (a) locating a containment means on top ofa support, (b) dosing a pre-determined quantity of a liquid componentinto said containment means, either in the order (a) then (b) or (b)then (a), and (c) by applying pressure or vacuum, drawing said liquidcomponent into at least a portion of the support, and retainingsubstantially all of said quantity within the support. This coatedproduct (coated from one end only) is dried and then calcined and thisprocess is repeated from the other end so that substantially the entiresubstrate monolith is coated, with a minor overlap in the axialdirection at the join between the two coatings. A core of 1 inch (2.54cm) diameter×3 inches long was cut from the finished article.

Comparative Example 2 Preparation of Pt-Only Catalysed Wall-Flow Filter

A washcoat composition comprising a mixture of alumina particles milledto a relatively high particle size distribution, platinum nitrate,binders and rheology modifiers in deionised water was prepared. Analuminium titanate wall-flow filter was coated with the catalystcomposition at a washcoat loading of 0.2 g/in³ to a final total Ptloading of 5 g/ft⁻³ using the method and apparatus disclosed in theApplicant/Assignee's WO 2011/080525, wherein channels at a first endintended for orientation to an upstream side were coated for 75% oftheir total length with a washcoat comprising platinum nitrate andparticulate alumina from the intended upstream end thereof; and channelsat an opposite end and intended to be oriented to a downstream side arecoated for 25% of their total length with the same washcoat as the inletchannels. That is, the method comprised the steps of: (i) holding ahoneycomb monolith substrate substantially vertically; (ii) introducinga pre-determined volume of the liquid into the substrate via open endsof the channels at a lower end of the substrate; (iii) sealinglyretaining the introduced liquid within the substrate; (iv) inverting thesubstrate containing the retained liquid; and (v) applying a vacuum toopen ends of the channels of the substrate at the inverted, lower end ofthe substrate to draw the liquid along the channels of the substrate.The catalyst composition was coated on filter channels from a first end,following which the coated filter was dried. The dried filter coatedfrom the first end was then turned and the method was repeated to coatthe same catalyst to filter channels from the second end, followed bydrying and calcining.

A core of 1 inch (2.54 cm) diameter×3 inches (7.62 cm) long was cut fromthe finished article. The resulting part is described as “fresh”, i.e.unaged.

Example 3 Preparation of Pt-Inlet/Pd-Outlet Containing CatalysedWall-Flow Filter

A coated filter was prepared using the same method as in ComparativeExample 2, except in that 100% of the total channel length of channelsintended for orientation towards the inlet side of gas contact wascoated with a washcoat containing platinum nitrate and alumina beforethe coated filter was dried; and 35% of the total length of channels ofthe Pt-coated filter intended for orientation towards the outlet sidewere coated with a washcoat containing palladium nitrate and alumina.The resulting fully coated filter was then dried, then calcined. Thetotal loading of Pt on the coated filter was 5 gft⁻³ and the totalloading of Pd on the coated filter was 1.75 gft⁻³.

A core of 1 inch (2.54 cm) diameter×3 inches long was cut from thefinished article. The resulting part is described as “fresh”, i.e.unaged.

Example 4 Preparation of Pt-Inlet/Al₂O₃-Outlet Containing CatalysedWall-Flow Filter

A coated filter was prepared using the same method as Example 3, exceptin that 35% of the total length of channels intended for orientationtowards the outlet side were coated with a washcoat containing aluminaonly. The resulting coated filter was then dried, then calcined. Thetotal loading of Pt on inlet channels of the coated filter was 5 gft⁻³.

A core of 1 inch (2.54 cm) diameter×3 inches long was cut from thefinished article. The resulting part is described as “fresh”, i.e.unaged.

Example 5 Preparation of Pt-Inlet/Single Layer Pt:Pd-Outlet ContainingCatalysed Wall-Flow Filter

A coated filter was prepared using the same method as in ComparativeExample 2, except in that the washcoat applied to the outlet channels ofthe filter included palladium nitrate in addition to the platinumnitrate. The washcoat loading in the inlet and outlet channels wasconducted in such a way as to arrive at a 5 g/ft³ Pt, 1.25 g/ft³ Pdloading on both the inlet surfaces and the outlet surfaces, i.e. a totalPGM loading of 6.25 g/ft³.

A core of 1 inch (2.54 cm) diameter×3 inches long was cut from thefinished article. The resulting part is described as “fresh”, i.e.unaged.

Example 6 Preparation of Pt-Inlet/Layered Pt/Pd-Outlet ContainingCatalysed Wall Flow Filter

A coated filter was prepared using the same method as in ComparativeExample 2, except in that two layers of washcoat were applied to the 25%total zone length of the outlet channels. In a first (or bottom) layer,the washcoat contained platinum nitrate and alumina. The coated filterwas then dried and calcined before a second (or top) layer washcoat wasapplied which contained palladium nitrate and alumina. The washcoatloading in the inlet and outlet channels was conducted in such a way asto arrive at a total combined loading on the inlet channels and theoutlet channels of 5 g/ft³ Pt, 1.25 g/ft³ Pd loading, i.e. a total PGMloading of 6.25 g/ft³.

A core of 1 inch (2.54 cm) diameter×3 inches long was cut from thefinished article. The resulting part is described as “fresh”, i.e.unaged.

Example 7 Preparation of 1:1 Weight % Pt:Pd Containing CatalysedWall-Flow Filter

A coated filter was prepared using the same method as in ComparativeExample 2, except in that the washcoat applied to both the inletchannels and the outlet channels of the filter included palladiumnitrate in addition to the platinum nitrate. The washcoat loading in theinlet and outlet channels was conducted in such a way as to arrive at a5 g/ft³ Pt, 5 g/ft³ Pd loading on both the inlet surfaces and the outletsurfaces, i.e. a total PGM loading of 10 g/ft³.

A core of 1 inch (2.54 cm) diameter×3 inches long was cut from thefinished article. The resulting part is described as “fresh”, i.e.unaged.

Example 8 Preparation of 5:1 Weight % Pt:Pd Containing CatalysedWall-Flow Filter

A coated filter was prepared using the same method as in ComparativeExample 2, except in that the washcoat applied to both the inletchannels and the outlet channels of the filter included palladiumnitrate in addition to the platinum nitrate. The washcoat loading in theinlet and outlet channels was conducted in such a way as to arrive at a5 g/ft³ Pt, 1 g/ft³ Pd loading on both the inlet surfaces and the outletsurfaces, i.e. a total PGM loading of 6 g/ft³.

A core of 1 inch (2.54 cm) diameter×3 inches long was cut from thefinished article. The resulting part is described as “fresh”, i.e.unaged.

Example 9 System Tests

The tests were performed on a first synthetic catalyst activity test(SCAT) laboratory reactor illustrated in FIG. 1, in which a fresh coreof the coated Fe/Beta zeolite SCR catalyst of Example 1 is disposed in aconduit downstream of a core of either the catalysed wall-flow filter ofComparative Example 2 or of Example 3, 4, 5, 6, 7 or 8. A synthetic gasmixture was passed through the conduit at a catalyst swept volume of30,000 hr⁻¹. A furnace was used to heat (or “age”) the catalysedwall-flow filter sample at a steady-state temperature at a filter inlettemperature of 900° C. for 60 minutes, during which the inlet SCRcatalyst temperature was 300° C. An air (heat exchanger) or watercooling mechanism was used to effect the temperature drop between thefilter and the SCR catalyst. The gas mixture during the ageing was 10%O₂, 6% H₂O, 6% CO₂, 100 ppm CO, 400 ppm NO, 100 ppm HC as Cl, balanceN₂.

Following ageing, the aged SCR catalysts were removed from the firstSCAT reactor and inserted into a second SCAT reactor specifically totest NH3-SCR activity of the aged samples. The aged SCR catalysts werethen tested for SCR activity at 150, 200, 250, 300, 350, 450, 550 and650° C. using a synthetic gas mixture (O₂=14%; H₂O=7%; CO₂=5%; NH₃=250ppm; NO=250 ppm; NO₂=0 ppm; N₂=balance) and the resulting NO_(x)conversion for Examples 3, 5 and 6 were plotted against temperature foreach temperature data point in FIG. 2 against fresh SCR catalystactivity and against an SCR catalyst aged behind Comparative Example 2.The graph shown in FIG. 3 plots the resulting NO_(x) conversion forExamples 4 and 7 using the same comparisons. This plot essentiallymeasures competition between reaction (9) and reaction (5) and thus howmuch reaction (9) affects the NO_(x) conversion by consumption of theavailable NH₃ needed for the SCR reaction (reaction (5)).

The results are shown in FIGS. 2 and 3. It can be seen that the SCRcatalysts for use in the exhaust system according to the presentinvention retain more activity than the SCR catalyst in ComparativeExample 2, although they retain less SCR activity than a fresh catalyst.The inventors interpret this result as showing that the loss in SCRactivity is caused in part by the deposition of low levels of Pt fromthe upstream catalysed wall-flow filter on the downstream SCR catalyst.Substantially no loss in activity was seen between a fresh Fe/Betacatalyst and a Fe/Beta catalyst aged at 300° C. for 1 hour without anycatalyst present upstream (results not shown).

Example 10 Preparation of Substrate Monolith Coated with 3 wt % Cu/CHAZeolite

Commercially available aluminosilicate CHA zeolite was added to anaqueous solution of Cu(NO₃)₂ with stirring. The slurry was filtered,then washed and dried. The procedure can be repeated to achieve adesired metal loading. The final product was calcined. After mixing,binders and rheology modifiers were added to form a washcoatcomposition.

A 400 cpsi cordierite flow-through substrate monolith was coated with anaqueous slurry of the 3 wt % Cu/CHA zeolite sample using the methoddisclosed in Applicant/Assignee's WO 99/47260 described in Example 1hereinabove. The coated substrate monolith was aged in a furnace in airat 500° C. for 5 hours. A core of 1 inch (2.54 cm) diameter×3 incheslong (7.62 cm) was cut from the finished article.

Example 11 Further Pt:Pd Weight Ratio Studies

Two diesel oxidation catalysts were prepared as follows:

Diesel Oxidation Catalyst A

A single layered DOC was prepared as follows. Platinum nitrate andpalladium nitrate were added to a slurry of silica-alumina. Beta zeolitewas added to the slurry such that it comprised <30% of the solidscontent as zeolite by mass. The washcoat slurry was dosed onto a 400cpsi flow-through substrate using the method of Example 1 hereinabove.The dosed part was dried and then calcined at 500° C. The total platinumgroup metal loading in the washcoat coating was 60 gft⁻³ and the totalPt:Pd weight ratio was 4:1. A core of 1 inch (2.54 cm) diameter×3 inches(7.62 cm) long was cut from the finished article. The resulting part maybe described as “fresh”, i.e. unaged.

Diesel Oxidation Catalyst B

A single layered DOC was prepared as follows. Platinum nitrate andpalladium nitrate were added to a slurry of silica-alumina. Beta zeolitewas added to the slurry such that it comprised <30% of the solidscontent as zeolite by mass. The washcoat slurry was dosed onto a 400cpsi flow-through substrate using the same method as used for DOC A. Thedosed part was dried and then calcined at 500° C. The total PGM loadingin the single layer DOC was 120 g/ft³ and the Pt:Pd weight ratio was2:1. A core of 1 inch (2.54 cm) diameter×3 inches (7.62 cm) long was cutfrom the finished article. The resulting part may be described as“fresh”, i.e. unaged.

Both catalysts were tested according the protocols set out in Example12. The results are set out in FIG. 5 with reference to a control (agedSCR catalyst that has not been further aged downstream of either DOC Aor DOC B).

Example 12 System Tests

The tests were performed on a first synthetic catalyst activity test(SCAT) laboratory reactor illustrated in FIG. 1, in which an aged coreof the coated Cu/CHA zeolite SCR catalyst of Example 10 was disposed ina conduit downstream of a core of either the Diesel Oxidation Catalyst(DOC) A or B (according to Example 11). A synthetic gas mixture waspassed through the conduit at a rate of 6 litres per minute. A furnacewas used to heat (or “age”) the DOC samples at a steady-statetemperature at a catalyst outlet temperature of 900° C. for 2 hours. TheSCR catalyst was disposed downstream of the DOC sample and was held at acatalyst temperature of 300° C. during the ageing process by adjustingthe length of tube between the furnace outlet and the SCR inlet,although a water cooled heat exchanger jacket could also be used asappropriate. Temperatures were determined using appropriately positionedthermocouples (T₁ and T₂). The gas mixture used during the ageing was40% air, 50% N₂, 10% H₂O.

Following the DOC ageing, the SCR catalysts were removed from the firstSCAT reactor and inserted into a second SCAT reactor specifically totest NH₃—SCR activity of the aged samples. The SCR catalysts were thentested for SCR activity at 500° C. using a synthetic gas mixture(O₂=10%; H₂O=5%; CO₂=7.5%; CO=330 ppm; NH₃=400 ppm; NO=500 ppm; NO₂=0ppm; N₂=balance, i.e. an alpha value of 0.8 was used (ratio ofNH₃:NO_(x)), so that the maximum possible NO_(x) conversion availablewas 80%) and the resulting NO_(x) conversion was plotted againsttemperature on the accompanying bar chart in FIG. 5. This plotessentially measures competition between reaction (9) and reaction (5)and thus how much reaction (9) affects the NO_(x) conversion byconsumption of the available NH₃ needed for the SCR reaction (reaction(5)).

Pt:Pd Weight Ratio Study—Conclusions

Taken as a whole, the results of Example 9 shown in FIG. 4 in connectionwith Examples 7 and 8 and Comparative Example 2 indicate that a Pt:Pdweight ratio of between 1:1 and 5:1 is beneficial in reducing theproblem of NO_(x) conversion activity loss through volatilisation ofplatinum group metal, principally platinum, from a platinum group metalcontaining catalyst to a downstream SCR catalyst; and

The results of Example 12 shown in FIG. 5 in connection with DieselOxidation Catalysts A and B show that for a SCR catalyst aged downstreamof a DOC having a 2:1 Pt:Pd weight ratio overall, the loss of NO_(x)conversion activity is relatively slight at 67% NO_(x) conversionactivity compared with the control at 72% NO_(x) conversion activity (aSCR catalyst aged behind a 1:1 Pt:Pd weight ratio overall DOC (notdescribed herein) using the same protocol had a NO_(x) conversionactivity of 69%). However, when the overall Pt:Pd weight ratio wasincreased to 4:1, SCR activity was significantly reduced to 48%.

The inventors conclude, therefore, that there exists a boundary at about2:1 Pt:Pd weight ratio overall above which Pt volatilisation is morelikely to occur. Hence, by limiting to an overall Pt:Pd weight ratio of2:1 in the DOC as a whole, and to ≦2:1 Pt:Pd weight ratio in the secondwashcoat coating layer, Pt in the DOC is less likely to volatilise andmigrate to a downstream SCR catalyst.

For the avoidance of any doubt, the entire contents of any and alldocuments cited herein is incorporated by reference into the presentapplication.

1. A catalysed substrate monolith for use in treating exhaust gasemitted from a lean-burn internal combustion engine, which catalysedsubstrate monolith comprising a first washcoat coating and a secondwashcoat coating, wherein the first washcoat coating comprises acatalyst composition comprising at least one platinum group metal (PGM)and at least one support material for the at least one PGM, wherein atleast one PGM in the first washcoat coating is liable to volatilise whenthe first washcoat coating is exposed to relatively extreme conditionsincluding relatively high temperatures, wherein the second washcoatcoating comprises at least one metal oxide for trapping volatilised PGMand wherein the second washcoat coating is oriented to contact exhaustgas that has contacted the first washcoat coating.
 2. The catalysedsubstrate monolith according to claim 1, wherein the at least one PGM inthe first washcoat coating comprises platinum.
 3. The catalysedsubstrate monolith according to claim 2, wherein the at least one PGM inthe first washcoat coating comprises both platinum and palladium
 4. Thecatalysed substrate monolith according to claim 3, wherein a weightratio of Pt:Pd is ≦2.
 5. The catalysed substrate monolith according toclaim 1, wherein the at least one metal oxide of the second washcoatcoating comprises a metal oxide selected from the group consisting ofoptionally stabilised alumina, amorphous silica-alumina, optionallystabilised zirconia, ceria, titania, an optionally stabilisedceria-zirconia mixed oxide and mixtures of any two or more thereof. 6.The catalysed substrate monolith according to claim 1, wherein thesecond washcoat coating comprises a catalyst composition comprising atleast one metal selected from the group consisting of palladium, silver,gold and combinations of any two or more thereof, wherein the at leastone metal oxide supports the at least one metal.
 7. The catalysedsubstrate monolith according to claim 5, wherein the second washcoatcoating comprises platinum and palladium, and wherein the weight ratioof Pt:Pd in the second washcoat coating is lower than the weight ratioof Pt:Pd in the first washcoat coating.
 8. The catalysed substratemonolith according to claim 1, wherein the first washcoat coating isdisposed in a first washcoat zone of the substrate monolith and thesecond washcoat coating is disposed in a second washcoat zone of thesubstrate monolith, wherein there is substantially no overlap betweenthe first washcoat zone and the second washcoat zone.
 9. The catalysedsubstrate monolith according to claim 1, wherein the second washcoatcoating is disposed in a layer above the first washcoat coating.
 10. Thecatalysed substrate monolith according to claim 1, wherein the substratemonolith is a flow-through substrate monolith.
 11. The catalysedsubstrate monolith according to claim 1, wherein the substrate monolithis a filtering substrate monolith having inlet surfaces and outletsurfaces, wherein the inlet surfaces are separated from the outletsurfaces by a porous structure.
 12. The catalysed substrate monolithaccording to claim 11, wherein the filtering substrate monolith is awall-flow filter.
 13. The catalysed substrate monolith according toclaim 12, wherein the first washcoat coating is disposed in a firstwashcoat zone of the substrate monolith and the second washcoat coatingis disposed in a second washcoat zone of the substrate monolith, whereinthere is substantially no overlap between the first washcoat zone andthe second washcoat zone, and wherein the first washcoat zone comprisesinlet surfaces of the filtering substrate monolith and the secondwashcoat zone comprises outlet surfaces of the filtering substratemonolith, wherein the porous structure defines a transition between thefirst washcoat zone and the second washcoat zone.
 14. The catalysedsubstrate monolith according to claim 1, wherein at least the firstwashcoat coating comprises an oxidation catalyst or a NO_(x) adsorbercatalyst.
 15. An exhaust system for a lean-burn internal combustionengine, which system comprising a first catalysed substrate monolithaccording to claim
 1. 16. The exhaust system according to claim 15,comprising a second catalysed substrate monolith comprising a selectivecatalytic reduction (SCR) catalyst, which second catalysed substratemonolith is disposed downstream from the first catalysed substratemonolith.
 17. The exhaust system according to claim 16, comprising aninjector for injecting a nitrogenous reductant into exhaust gas betweenthe first catalysed substrate monolith and the second catalysedsubstrate monolith.
 18. The exhaust system according to claim 15,comprising a third substrate monolith, wherein the third substratemonolith is a filtering substrate monolith, which third substratemonolith is disposed downstream of the second catalysed substratemonolith.
 19. The exhaust system of claim 18, wherein the thirdsubstrate monolith comprises an oxidation catalyst.
 20. A method ofreducing or preventing a selective catalytic reduction (SCR) catalyst inan exhaust system of a lean-burn internal combustion engine frombecoming poisoned with platinum group metal (PGM) which may volatilisefrom a catalyst composition comprising at least one PGM supported on atleast one support material and disposed on a substrate monolith upstreamof the SCR catalyst when the catalyst composition comprising PGM isexposed to relatively extreme conditions including relatively hightemperatures, which method comprising trapping volatilised PGM in awashcoat coating comprising at least one metal oxide, which is disposedon the same substrate monolith as the catalyst composition comprisingPGM.